Abstract

Phlogopite-dolomite-peridotite is
the most promising source rock for kimberlites
and related magmas. At pressures above about
30 kb, very little CO_2 (low CO_2/H_2O in vapor) is
required to produce dolomite in mantle peridotite.
If oxygen fugacity is too low, however, CO_2 and
carbonate are reduced to carbon, and dolomite is
unable to exert its distinctive influence on
magma compositions. The oxygen fugacity at
various depths in the mantle is a critical
factor. Rare diamonds and even rarer carbonates
occur in peridotite nodules from kimberlite, and
CO_2 is brought to the surface in mantle-derived
minerals and lavas. Phase relationships in
peridotite-CO_2-H_2O provide a first step for
evaluation of the behavior of components C-H-O
at depth. Experimental and theoretical data
from various sources have been combined for
analysis of the near-solidus phase relationships
in peridotite-CO_2-H_2O. The divariant solidus
surface is traversed by a series of univariant
lines where the vapor phase is buffered by
amphibole, dolomite (magnesite at higher press
ures), phlogopite, or combinations of these.
The lines limit the range of vapor-phase compositions that can coexist with peridotite at
various pressures. The buffering capacity of
dolomite is far greater than that of the hydrous
minerals. The buffered curves for partly carbonated peridotite, with and without phlogopite,
extend to lower temperatures and higher pressures
from an invariant point near 26 kb and
1200°C. Near this line there is a temperature maximum (a ridge) on the solidus surface,
separating the low-pressure surface, where CO_2/H_2O in vapor is higher than in liquid, from the
high-pressure surface, where CO_2/H_2O in vapor is
lower than in liquid. Enrichment of the high-pressure
liquids in CO_2 is associated with the
generation of dolomite and low- SiO_2 liquids.
Because of this maximum on the solidus, near-solidus
magmas rising along an adiabat would
evolve volatile components in the depth interval
100-80 km, which could contribute to the explosive eruption of kimberlites. The subcontinental
upper mantle is probably heterogeneous with
respect to incompatible elements, because local
melting due to sparsely distributed CO_2 and H_2O
(dolomite and phlogopite) is followed by magmatic
flushes, as the melt migrates upwards.